A pressure swing adsorption (PSA) process for producing the less strongly adsorbed component of a feed gas mixture which utilizes a first adsorbent for bulk removal of the more strongly adsorbed component and a second adsorbent for trace removal of the more strongly adsorbed component is taught in the art. Specifically, Japanese Laid-Open Patent Application No. 63-240914 by Takahashi et al. teaches such a process in the context of:
(1) an air feed comprising oxygen as the more adsorbable component and nitrogen as the less adsorbable component; PA1 (2) a kinetically controlled adsorbent (such as a carbon molecular sieve adsorbent) as the first adsorbent for bulk removal of the oxygen; and PA1 (3) an equilibrium control led adsorbent (such as a metal complex-based adsorbent) as the second adsorbent for trace removal of the oxygen.
A key advantage of such a dual adsorbent design is that because the kinetically controlled adsorbent accomplishes the bulk of the oxygen removal, the rapid kinetics and high selectivity of the equilibrium controlled adsorbent can be utilized for trace oxygen removal without subjecting the equilibrium adsorbent to destructively high oxygen concentrations. This, in turn, allows a very high purity nitrogen product (less than 1000 ppm oxygen, preferably less than 100 ppm oxygen) to be produced. Heretofore, such high nitrogen purities generally required cryogenic technology (which is very capital and energy intensive) or deoxo technology (which, because it uses hydrogen to regenerate the catalyst/adsorbent, requires costly safety control systems in addition to producing a hydrogen and water vapor contaminated product).
There are concerns, however, with regard to Takahashi's single stage design whereby the adsorbents are layered in a single bed. In an ideal situation where the equilibrium controlled adsorbent's stability is comparable to the kinetically controlled adsorbent's stability, Takahashi's layered bed design is desirable. Unfortunately, most of the current equilibrium controlled adsorbents are not as stable as kinetically controlled adsorbents and thus have a higher replacement frequency which makes Takahashi's layered bed design undesirable. In addition, such a dual stage design lacks the flexibility for independent optimization of each layer's operating conditions. The present invention overcomes these concerns by using two stages whereby, for example, the kinetically controlled adsorbent is contained in the first stage and the equilibrium controlled adsorbent is contained in the second stage. In addition to accommodating the equilibrium controlled adsorbent's higher replacement frequency, such a dual stage design also provides the flexibility for independent optimization of each stage's operating conditions.
The present invention also provides for the efficient integration of the two stages in that the desorbed gas which is produced during depressurization of the second stage is recycled to the first stage (as additional feed for example) to improve the performance of the first stage. The reason that such a recycle scheme improves the performance of the first stage is that, because the first stage will already have removed most of the more adsorbable component from the fresh feed to the process, the desorbed gas from the second stage will consist primarily of the desired less adsorbable component. Indeed, the concentration of the desired component will be higher in the desorbed gas from the second stage vis-a-vis the fresh feed to the process. Thus, by recycling the second stage desorbed gas to the fresh feed to the process, the concentration of the desired component in the feed to the first stage is made higher than it would be otherwise. This gives the first stage a "head start" in concentrating the desired component which improves the performance of the first stage. The closest art to such recycling of the second stage desorbed gas is probably UK Patent 2,227,685 by Garrett et al. which teaches a single stage adsorption process wherein the desorbed gas from the latter part of the depressurization step is recycled to the feed.